Demodulation circuit



Aug. 17, 1954 w, w, MQE 2,685,870

DEMODULATION CIRCUIT Filed Aug. 13. 1949 -MODUL/ITED SIGNAL SOURCE O O15 /4' I7 22 2 RECTIFIER 3 LOAD (r 7 /6 24 i T T" /2 I JNI'ENTOR.

WILLIAM WEST MOE HIS ATTORNEYS.

Patented Aug. 17, 1954 emce DEMODULA TION CIRCUIT William West Moe,Strati Time; Incorporated, Ne

ration of N ew York 0rd,, Conn assignou to, w York, N. Y.', a. .corpo-This invention relates to filter networks and more specifically to anewandimproved filter network which is of special utility for filtering outcomponents or a carrier frequencylfrom a rectified modulated carrier,where there are modulation components whose frequency isnear the car:-rier'frequency.

In'certain applications of electronic circuits, the necessitysometimeslarises for eiiective filtering of carrier frequency componentsfrom a rectifled modulated carrier, where the modulation i m pressedupon the carrier includes components of frequencies near the carrierfrequency. Thus, in electronic color correctioncircuits a carrierfrequency of say 7,680cyc1es" may be modulated by picture components inthe, range from zero to 1,000 cycles; The carrieryis subsequentlysubjected to rectificationand components of un Wantedfrequenciesarefiltered out leaving a slgnal varying in accordance withthe modulation." In order to obtain a signal thataccurately representsthe modulationimpressed on *the carrier, the rectified carrier should befed into a large capacitor so that a D. C. voltage equal to the peak A.C. voltage would-be produced. Ifthis-isdone, however, some of the highermodulation components will be'lost through the low impedance shunt pathformed-by the condenser.-

It is an object of theinvention, accordingly, to provide a new andimproved filter circuit which is capable of producing" a D; C; voltagesubs-tan tially equal to the peak AC. voltage in a system" of this typeWithout-losing higher-modulation components.

Another object of the invention is to provide a newand improvedfiltercircuit of the above character in which the D. C: signal output willfollow rapid variations -in theA. C; modulated carrier.

According to the inventionya filter circuit is provided in which seriesresonant circuits tuned to the more important lower frequency-even"harmonies are employed for eliminating thecarrier and its', harmonics.lffull waverectification is employed, it has been found: that two seriesresonant circuits, oneituned to twice thecarrier frequency and one tuned:to fourxtimesthe carrier frequency, are sufficient to raise:ltheD..C.output voltage to within a few'perl cent cfgthezpeak 'A.=C. inputvoltage. Thelefiect of the tuned circuitslat modulation frequenciesisessentially?theisameaas lower frequency even"harmonics;

I across the 2;; thatof a very small shuntcapacity. which forms the.firstscapacitancel of the low pass .filter. AC:- cordingly, the.D...C.output .will .follow rapid variations in then. C. modulatedcarrier.without material loss of the higher .modulation components that. aredeveloped The invention may Joebetterl understood from the-.iollowingdetailed description of a representae tivelembodiment thereof, taken inconjunction with thesingleaccompanying drawing,

Referring now to the single figure, a modulated signal source is shownat l 0 which provides an output comprising a carrier of say 7,68O'cyclesper second modulated by signals in; the frequency range of say zero to1,000cycles per second. The modulated signaloutput from thesource i9 issupplied through the. conductors l I; to a convene tional full waverectifier |2-,- the output from which includes modulation components aswell as thecarrier and its'harmom'cs.

As indicated, in order to obtain a D. C. voltage substantially equal ,tothe peak, A. C. voltage, the output'fromthe full wave rectifier I22should be fed to, a large, capacitor: ,This cannot be done, however,becausea large capacitor would act as a lowimpedance shunt to the highermodulation components ,whichare developed. Since the A. C. components inthe output voltage'from the full wave rectifier Ill are 'predominantlylower even harmonics of the 7,680cyc1espersecond carrier frequency, theeffect of a large capacitor, so far as the A.C.'harmonics are concerned,is produced according to the invention by using series resonantelectrical circuitstuned to the important It has been found thattwoseries resonantcircuits; one tuned todouble'thecarrier frequency, andone tuned to four times the carrier frequencyaresuflicient to producethe desired D. C. voltage output.

'Ih'us,-the-output from the full wave rectifier I 2 isfed through theconductors l 3to a first series resonant-circuit ld comprisingarr-inductance i5, and a capacitance l l 6, which i s-tuned to twicethecarrier frequency. Connected in parallel with the seriesresonant=circuit i4: is a second series resonantrcircui-t: Illincludinganrinductance l8 anda capacitance: t9; whichistunedto fourtimes thecarrienufrequency; Thewoutput, appearing parallel yconnected: tuned,circuits 1 6 audit! i is supplied to, a conventional low pass filtercomprising; the: series. 'inductances 2c and 2 l and 3 the capacitances22, 23 and 24, the final filtered output being supplied to a load 25.

For a carrier frequency of 7,680 cycles containing modulation componentsin the frequency range of zero to 1,000 cycles, the desired filteringeffect can be secured with the following typical values for the severalcircuit components:

Inductance l5 .25 henry, approximately. Inductance I8 .15 henry,approximately. Inductance 20 henries.

Inductance 2i 10 henries.

Capacitance l6 350 mmf.

Capacitance I9 -s 190 mmf.

Capacitance 22 .001 mfd.

Capacitance 23 '75 mmf.

Capacitance 24 500 mmf.

It will be understood from the foregoing that the invention provides ahighly effective filter circuit which is capable of producing a D. C.signal that follows rapid variations in the A. C. voltage without anyappreciable loss of the higher frequency modulation components which aredeveloped. At modulation frequencies, the tuned circuits used inaccordance with the invention behave very much like a small shuntcapacity which forms the first capacitance of the succeeding low passfilter which is employed to pass modulation components and to eliminatethe carrier and its harmonics.

The illustrative carrier and modulation frequencies and the typicalvalues for the several circuit components given above are intendedmerely to be illustrative. Obviously, they can be varied over a widerange depending upon the results desired. Also, other. modifications maybe made in the circuit within the spirit of the invention, as will beapparent to those skilled in the art. The specific embodiment describedabove and illustrated in the drawings is not to be regarded, therefore,as restricting in any way the scope of the appended claims.

I claim:

1. A demodulation circuit for passing modulation components in amodulated carrier having modulation components near the carrierfrequency and for eliminating the carrier and unwanted harmonicsthereof, comprising, means for rectifying said modulated carrier toprovide the rectified modulated carrier as an output, a series resonantcircuit connected to receive said rectified modulated carrier fordemodulation thereof tuned to the second harmonic of said carrier, andseries inductance-shunt capacitance filter means having an inputconnected across said series resonant circuit, said filter meansincluding a series inductance branch interposed between said seriesresonant circuit and any shunt capacitance of said filter means, saidfilter means being tuned to pass modulation compo nents to an output ofsaid filter means and to eliminate the carrier and unwanted harmonicsthereof.

2. A demodulation circuit for passing modulation components in amodulated carrier having modulation components near the carrierfrequency and for eliminating the carrier and unwanted harmonicsthereof, comprising, means for rectifying said modulated carrier toprovide the rectified modulated carrier as an output, a

first series resonant circuit connected to receive said rectifiedmodulated carrier and tuned to the second harmonic of said carrier, asecond series resonant circuit connected directly in shunt with saidfirst resonant circuit and tuned to a higher harmonic of said carrier,said series resonant circuits being adapted cooperatively to effectdemodulation of said rectified modulated carrier, and seriesinductance-shunt capacitance filter means having an input connectedacross said second series resonant circuit, said filter means includinga series inductance branch interposed between said series resonantcircuits and any shunt capacitances of said filter means, said filtermeans being tuned to pass modulation components to an output of saidfilter means and to eliminate the carrier and harmonics thereof.

3. A demodulation circuit for passing modulation components in amodulated carrier having modulation components near the carrierfrequency and for eliminating the carrier and other unwanted harmonicsthereof, comprising, means for full-wave rectifying said modulatedcarrier to provide the rectified modulated carrier as an output a firstseries resonant circuit connected to receive said rectified modulatedcarrier and tuned to the second harmonic thereof, a second seriesresonant circuit connected directly in shunt with said first resonantcircuit and tuned to the fourth harmonic of said carrier said seriesresonant circuits being adapted cooperatively to effect demodulation ofsaid rectified modulated carrier, and two stage, series inductance-shuntcapacitance filter means having an input connected across said secondresonant circuit said filter means including a series inductance branchinterposed between said series resonant circuits and any shuntcapacitances of said filter means, said filter means being tuned to passmodulation components to an output of said filter means and to eliminatethe carrier and unwanted harmonies thereof.

l. Demodulation apparatus for demodulating a modulated carrier signalwhere the modulation impressed upon the carrier includes components offrequencies near the carrier frequency, comprising, means for rectifyingsaid modulated carrier to provide the rectified modulated carrier as anoutput inductance input filter means having an output and includingseries inductance means and shunt capacitance means in a mutual networkrelation wherein at least some of said series inductance means isinterposed between said shunt capacitance means and said input, saidfilter means being designed for attenuation of carrier signal componentsin said rectified signal when passing through said filter means to saidoutput without appreciable attenuation of modulation components thereof,and first and second series resonant circuits adapted to receive saidrectified modulated carrier and both connected directly in shunt withthe input of said filter means, said first and second resonant circuitsbeing tuned, respectively, to the second and another major harmonic ofsubstantial magnitude which are present in said rectified carrier signalfor demodulation of said rectified modulated carrier signal, whereby theform of the filtered output voltage from the apparatus will besubstantially identical with the modulation envelope of the originalcarrier signal.

5. Demodulation apparatus as defined in claim l for demodulating saidmodulated carrier signal in which said first and second series resonantcircuits are tuned to the second and fourth harmonics, respectively, ofthe carrier frequency.

6. A demodulation circuit for demodulating a modulated carrier havingmodulation components near the carrier frequency comprising, means forrectifying said modulated carrier to provide the rectified modulatedcarrier as an output, a pair of series resonant circuits connecteddirectly in parallel to receive the rectified modulated carrier to befiltered, and tuned to the second and another higher major harmonic ofthe carrier frequency for demodulation of the rectified modulatedcarrier, and series pacitance filter means having an input connectedacross said resonant circuits, said filter means including a seriesinductance branch interposed between said series resonant circuits andany shunt capacitance of said filter means, said filter means beingtuned to pass some modulation components to an output of said filtermeans and to reject others.

inductance-shunt ca- References Cited in the file of this patent UNITEDSTATES PATENTS Number Name Date Gerth -1 Apr. 14, 1931 Crouse Aug. 1,1933- Posthumus et a1. July 31, 1934 Roberts Aug. 14, 1934 Tuttle Aug.27, 1935 Rose Sept. 24, 1935 Wright Mar. 1, 1938 Salzberg Feb. 10, 1942Magnuski Aug. 26, 1952

